Methods for producing "hot-laid"bitumen-subdivided solid mineral matter mixtures are known in the art. In such prior art methods, the mineral matter, which generally has a moisture content of about 2% to about 6%, is heated to from about 120.degree. C to about 250.degree. C, for example in a drum apparatus, wherein it is dried prior to intermixture with the bitumen.
Subsequent to the drying step, the subdivided mineral components, which comprise a desired range of grain sizes, are preferably fed to a charge or batch mixer in which the mineral components are intermixed with bituminous binder components and possibly also with additives such as adhesives and the like. Additional mineral matter in the form of mineral dust or flour is introduced into the batch mixer where such addition is necessary for maintaining the desired mesh size distribution of the mineral matter mixture.
The addition of mineral dust or flour is especially required because during the drying the mineral components of fine grain size naturally present in the mineral matter mixture tend to be separated from the granules of coarser grain size. Such separation cannot be avoided due to the fact that hot gases are required for the drying step. The result of such separation is that there is a lack of the finest grain size in the mixture, which is especially disadvantageous in a mixture to be used for producing so-called asphalt concrete. Such lack of the finest grain size components must be compensated for by adding predetermined amounts of so-called foreign and/or feed back mineral matter having the desired finest grain size.
The above-described method which is generally employed in the art has several drawbacks.
First, it is imposible to avoid the large dust formation during the drying step. As a result, it is necessary to coordinate with the drying equipment, dust collection devices at substantial expense.
Second, it is necessary to handle the dust collected in the dust collection devices because such dust can be reused. Such handling involves transporting, storing and metering the dust as well as feeding the dust back in proper quantities to the material to be mixed. Due to the storage of the dust, it is impossible to avoid a temperature drop in the dust. Such temperature drop makes it rather difficult to achieve a flawless mixing with the bituminous binder components.
Third, it is quite frequently insufficient merely to return the amounts collected in the dust collection devices so that a make-up addition of mineral dust becomes necessary.